在 Windows Phone 8 中异步下载和保存文件 [英] Downloading and saving a file Async in Windows Phone 8

问题描述

如何将文件异步下载并保存到 IsolatedStorage?我首先使用 WebClient 来达到目的，但我不能等到完成，所以我找到了一些有用的代码 此处.

How can I download and save a file to IsolatedStorage asynchronously? I used WebClient first for the purpose, but I couldn't await till completion so I found some useful code here.

然而，这也不是完整的格式.我找到了函数:

However, this is also not in complete format. I found the function:

public static Task<Stream> DownloadFile(Uri url)
{
    var tcs = new TaskCompletionSource<Stream>();
    var wc = new WebClient();
    wc.OpenReadCompleted += (s, e) =>
    {
        if (e.Error != null) tcs.TrySetException(e.Error);
        else if (e.Cancelled) tcs.TrySetCanceled();
        else tcs.TrySetResult(e.Result);
    };
    wc.OpenReadAsync(url);
    return tcs.Task;
}

但是我怎样才能把这个文件保存到IsolatedStorage?有人帮我完成这个功能！

But how can I save this file to IsolatedStorage? Someone help me to complete this functionality!

推荐答案

这个，har07 发布的内容应该可以工作.如果你需要稍微扩展你的方法，我会尝试做这样的事情(虽然它没有经过测试，但应该可以工作):

This, what har07 posted, should work. In case you need to extend your method a little, I would try to do something like this (though it is not tested, but should work):

(评论后重建 - 取消)

(Rebuilt after comment - with Cancellation)

// first define Cancellation Token Source - I've made it global so that CancelButton has acces to it
CancellationTokenSource cts = new CancellationTokenSource();
enum Problem { Ok, Cancelled, Other }; // results of my Task

// cancelling button event
private void CancellButton_Click(object sender, RoutedEventArgs e)
{
   if (this.cts != null)
        this.cts.Cancel();
}

// the main method - I've described it a little below in the text
public async Task<Problem> DownloadFileFromWeb(Uri uriToDownload, string fileName, CancellationToken cToken)
{
   try
   {
       using (Stream mystr = await DownloadFile(uriToDownload))
           using (IsolatedStorageFile ISF = IsolatedStorageFile.GetUserStoreForApplication())
           {
           if (ISF.FileExists(fileName)) return Problem.Other;
           using (IsolatedStorageFileStream file = ISF.CreateFile(fileName))
           {
               const int BUFFER_SIZE = 1024;
               byte[] buf = new byte[BUFFER_SIZE];

               int bytesread = 0;
               while ((bytesread = await mystr.ReadAsync(buf, 0, BUFFER_SIZE)) > 0)
               {
                  cToken.ThrowIfCancellationRequested();
                  file.Write(buf, 0, bytesread);
               }
           }
       }
       return Problem.Ok;
    }
    catch (Exception exc)
    {
       if (exc is OperationCanceledException)
           return Problem.Cancelled;
       else return Problem.Other; 
    }
}

// and download
private async void Downlaod_Click(object sender, RoutedEventArgs e)
{
   cts = new CancellationTokenSource();
   Problem fileDownloaded = await DownloadFileFromWeb(new Uri(@"http://filedress/myfile.txt", UriKind.Absolute), "myfile.txt", cts.Token);
   switch(fileDownloaded)
   {
      case Problem.Ok:
           MessageBox.Show("File downloaded");
           break;
      case Problem.Cancelled:
           MessageBox.Show("Download cancelled");
           break;
      case Problem.Other:
      default:
           MessageBox.Show("Other problem with download");
           break;
    }
}

我添加了取消令牌 - 这意味着您的下载操作可以在 Button.Click 之后取消.另一方面，如果 await DownloadFile(uriToDownload) 被取消，它会自动抛出 OperationCancelled - 然后您捕获该异常并返回足够的结果.

我还没有运行该代码，但它可能会显示主要思想.

I've added Cancellation Token - it means that your download operation can be cancelled after Button.Click. On the other hand if await DownloadFile(uriToDownload) is Cancelled it automatically throws OperationCancelled - then you catch that exception and return adequate result.

I haven't run that code, but it may show the main idea.

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